The present invention relates to a device and process for recovering powder and to an installation for spraying pulverulent coating product comprising such a device.
In an installation for spraying coating product, it is known to recover the powder which has not been deposited on the objects to be coated and which mostly drops towards the booth floor. The recovered powder is directed towards a sieving device which allows evacuation of the possible piles of powder, waste and dirt mixed with the recovered powder. A suction sheath is generally provided in which a mixture is formed between conveying air and the recovered powder, this mixture being displaced up to a centrifugation cyclone in which the powder particles are separated from the conveying air before being directed towards a sieve from which the recovered powder is re-injected into a circuit feeding coating product to the sprays of the installation.
Now, the velocity of the air/powder mixture inside the separation cyclone may be very high, in particular in the lower part of the cyclone, i.e. in its zone of smallest radius. The powder therefore hits the mesh of the sieve at a high velocity and it tends to rub on this mesh before being stabilized, such friction provoking heating which may lead to a melting and even sometimes a total or partial polymerization of the powder on the upper face of the sieve, in particular on the outer zone thereof. For example, in the case of a disc-shaped sieve of radius R, it is not rare for the sieve to be clogged over an annular surface of radius included between R/2 and R. Such clogging of the sieve reduces its performances and necessitates regular maintenance, this increasing exploitation costs of such an installation accordingly.
It is a particular object of the present invention to overcome these drawbacks by proposing a powder recovery device which avoids both an undesirable heating of the powder particles when they come into contact with the sieve and consequently clogging of the sieve.
To that end, the invention relates to a device for recovering powder in an installation for spraying pulverulent coating product, this device comprising:
at least one cyclone for separating the powder recovered in the installation and its conveying air, and
a sieve associated with the or each cyclone, characterized in that it comprises a perforated plate disposed on the path of the recovered powder, between the cyclone and the sieve.
The perforated plate considerably reduces the tangential velocity of the air/powder mixture, with the result that the velocity of this mixture is low, and even virtually zero, when it comes into contact with the sieve. No noteworthy heating is created.
According to a first advantageous aspect of the invention, the plate is made of a material having a low coefficient of friction with the powder, in particular polyethylene or polytetrafluoroethylene. Thanks to this aspect of the invention, no heating is created either, when contact is made between the powder particles of the air/powder mixture and the perforated plate.
According to another advantageous aspect of the invention, the plate is made of an electrically conducting material, which allows the electrical charge possibly created by tribo-electric effect when the particles of powder rub on the plate, to run off.
The plate is advantageously disposed above the mesh of the sieve, parallel thereto, at a distance of between 0.5 and 30 cm.
According to variant embodiments of the invention, the plate may be provided to be perforated over substantially the whole of its surface or to be in the form of a disc and perforated solely over an outer annular part.
Whatever the embodiment considered, the diameter of the perforations of the plate is advantageously included between 2 and 20 mm.
The invention also relates to an installation for spraying pulverulent coating product, which comprises a powder recovery device as described hereinabove. Such an installation operates with a better yield insofar as it is unnecessary to provide periodic unclogging of the sieve that it comprises, while the quality of the recovered powder remains optimal.
Finally, the invention relates to a powder recovery process which may be carried out with the device as described hereinabove and in which:
the powder recovered in a spray booth is conveyed towards a separation device,
the powder is separated from its conveying air by using centrifugal forces and the recovered powder is sieved. This process further consists in considerably reducing the tangential velocity of the powder before it arrives in the sieving zone, by causing the air/powder mixture to pass through a perforated plate.